Consumer and commercial gas burning appliances, such as ovens, clothes dryers, hot water heaters, etc., typically utilize a gas control flow valve that controls the flow of gaseous fuel to a burner. The appliance controller typically monitors user inputs and a temperature of the oven cavity, dryer drum, water storage tank, etc. to determine when a call for heat is required. Once the controller determines that a call for heat is required, the controller will open the gas control valve and energize the ignition circuitry at the burner to ignite the gaseous fuel being released therefrom. The controller continues to monitor the temperature of the oven cavity, dryer drum, water storage tank, etc. until it determines that a call for heat is no longer required. At such point, the controller turns off the gas control valve to extinguish the flame at the burner.
While the user typically sets a desired temperature set point for the appliance, and while the controller attempts to regulate the temperature at the user set point, the use of an on-off gas control valve limits the ability of the controller to maintain the temperature at the user selected set point. This is because the status of the burner is only controllable between full off and full burner on operating modes. When the burner is turned off, the temperature in the temperature regulated area of the appliance will drop based on ambient conditions and operation of the appliance. Once the temperature has dropped below a historicize level set by the controller, the burner is turned on. With the burner turned full on, the temperature rise in the appliance will be rather significant, at least compared to the temperature drop, in most appliance applications. Such on-off control, therefore, results in relatively wide and varying rate temperature deviations from the user set point.
To minimize these problems, some consumer and commercial appliances have begun to incorporate pulse width modulated gas control valves that can provide for a varying flow rate of gas to the burner. The availability of a variable gas flow rate based upon the duty cycle at which the pulse width modulated gas control valve is operated eliminates the wide temperature swings and heating and cooling rates that plague the on-off gas control mechanisms. Indeed, varying the duty cycle at which the gas control valve is modulated allows the appliance controller to vary the heating level from high to low to more precisely control the temperature of the appliance. In other words, by varying the relative amount of time that the modulated gas control valve is open and closed allows the controller to vary the amount of gas flowing to the burner between the maximum flow rate when the valve is open at the maximum duty cycle to a minimum flow rate when the valve is modulated at a minimum duty cycle.
While such pulse width modulation of the gas control valve provides a significant advantage over the previous on-off control in those appliance applications that need such more precise control, such pulse width modulation increases the complexity of the controller and associated drive circuitry, and therefore increases the cost over the prior on-off controls. Further, since the pulse width modulation operates to repeatedly open and close the gas control valve to regulate the flow of gas therethrough, the valve seat experiences increased wear due to the increased number of times that it is opened and closed.
Further, since the solenoid coil has an inductance associated therewith, the rate at which the current is changed will influence the coil voltage as governed by the equation V=L di/dt. Such modulation, therefore, will also increase the wear on the coil itself, and will limit the duty cycle at which the coil may be operated, thereby limiting the minimum amount of gaseous fuel that may be flowed therethrough. This flow rate limitation will also be influenced by the mechanical inertia that is inherent in the mechanical plunger that positions the valve seat in an opened or closed position. Additionally, such control strategies may increase the power consumption of the appliance due to the solenoid coil inrush current and because more time is spent overcoming the opening force of the valve seat than the on-off control, recognizing that the holding power requirement is much less than the opening power requirement of a typical valve.
In view of the above, there is a need in the art for a variable gas flow control valve that may be used in commercial and consumer appliances that better regulates the temperature thereof, but which does not suffer from the problems associated with pulse width modulated variable flow gas valves. Embodiments of the present invention provide such a variable flow rate gas control valve. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.